Plasma-resistant, welded aluminum structures for use in semiconductor apparatus

ABSTRACT

We have discovered a method of producing a complex-shaped aluminum alloy article, where welding has been employed to form the article, where an anodized aluminum coating is produced over a surface of the article including the weld joint, and where the anodized aluminum coating is uniform, providing improved performance over that previously known in the art for welded articles exposed to a corrosive plasma environment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

In general, the present invention relates to a method of fabrication ofsemiconductor processing apparatus from an aluminum substrate. Inparticular, the invention relates to a method of forming complex shapeswhich can subsequently be anodized to provide at least oneplasma-resistant surface, and particularly a surface which is resistantto halogen-containing plasmas.

2. Brief Description of the Background Art

Semiconductor processing involves a number of different chemical andphysical processes whereby minute integrated circuits are created on asubstrate. Layers of materials which make up the integrated circuit arecreated by chemical vapor deposition, physical vapor deposition andepitaxial growth, for example. Some of the layers of material arepatterned using photoresist masks and wet and dry etching techniques.Patterns are created within layers by the implantation of dopants atparticular locations. The substrate upon which the integrated circuit iscreated may be silicon, gallium arsenide, glass, or any otherappropriate material.

Many of the semiconductor processes used to produce integrated circuitsemploy halogen or halogen-containing gases or plasmas which areparticularly corrosive to processing apparatus surfaces they contact.Some processes use halogen-containing liquids. In addition, since theprocesses used to create the integrated circuits leave contaminantdeposits on the surfaces of the processing apparatus, such deposits arecommonly removed using plasma cleaning techniques which employ at leastone halogen-containing gas, which is also corrosive.

Aluminum has been widely used as a construction material forsemiconductor fabrication equipment, at times because of its conductiveproperties, and generally because of ease in fabrication andavailability at a reasonable price. However, aluminum is susceptible toreaction with halogens such as chlorine, fluorine, and bromine, toproduce, for example, AlCl₃ (or Al₂Cl₆); or AlF₃; or AlBr₃ (or Al₂Br₆).The aluminum-fluorine compounds can flake off the surfaces of processapparatus parts, causing an eroding away of the parts themselves, andserving as a source of particulate contamination of the process chamber(and parts produced in the chamber). Most of the compounds containingaluminum and chlorine and many of the compounds containing aluminum andbromine are gaseous under semiconductor processing conditions and leavethe aluminum structure, creating voids which render the structureunstable and with a surface having questionable integrity.

Porosity of the surface of aluminum semiconductor processing apparatusis of grave concern. We discovered that when a large block of aluminumalloy such as 6061 is machined out (hogged out) to produce a complexshape such as a liner for a processing vessel, the machined surfaceexhibits a high degree of porosity. We considered extruding a tube, as atechnique for obtaining an improved aluminum grain structure, and thenjoining the tube to a top plate to obtain a process chamber liner shape.However, then the problem shifts to joining of the tube to the topplate. Welding of the tube to the top plate typically creates impuritiesat the interface of materials coming together in the weld (at the jointof the weld), and the impurities frequently increase porosity at theweld joint. The impurities may be in the form of a filler material usedin the welding process or may be in the form of impurities present inthe aluminum alloy itself which migrate to the weld joint area duringthe welding process. Welding is generally defined as a coalescence ofmetals produced by heating to a suitable temperature with or without theapplication of pressure, and with or without the use of a fillermaterial. Some of the more commonly used welding techniques includeelectron-beam welding, laser welding, and solid-phase welding. Solidphase welding processes include, for example, diffusion bonding,friction welding, and ultrasonic joining. Solid phase processestypically produce welds without melting the base material and withoutthe addition of a filler material. Pressure is always employed, andgenerally some heat is provided. Furnace heating is generally providedin diffusion bonding, while frictional heat is developed in ultrasonicand friction joining.

Welding typically produces stress both at the welding joint and inmaterial adjacent the welding joint. Heat treatment or annealing iscommonly used to relieve stress. Aluminum alloys begin to exhibit graingrowth at temperatures approaching 345° C., which causes precipitationof non-aluminum metals at the grain boundaries. This precipitation maylead to cracking along a weld joint when the weld joint is mechanicallyloaded, and may lead to cracking along grain boundaries duringmachining. The precipitation also reduces mechanical properties of thealloy by affecting the uniformity of the alloy composition within thearticle.

If an aluminum alloy is to perform well in a number of semiconductorprocess apparatus applications, it should also have desirable mechanicalproperties. Further, mechanical properties should enable machining toprovide an article having the desired final dimensions. For example, ifthe alloy is too soft, it is difficult to drill a hole, as materialtends to stick during the drilling rather than to be removed by thedrill. Controlling the dimensions of the machined article is moredifficult. There is a penalty in machining cost. In addition, themechanical properties of the article affect the ability of the articleto perform under vacuum, depending on the function of the article. Forexample, a process chamber must exhibit sufficient structural rigidityand resistance to deformation that it can be properly sealed againsthigh vacuum.

With respect to resistance to a halogen-containing plasma, a preferredmeans of protection of the aluminum surfaces within a process apparatushas been an anodized aluminum coating. Anodizing is typically anelectrolytic oxidation process that produces an integral coating ofrelatively porous aluminum oxide on an aluminum surface. Despite the useof anodized aluminum protective layers, the lifetime of anodizedaluminum parts in semiconductor processing apparatus has been limited,due to the gradual degradation of the protective anodized film. Inaddition, in the past, the combination of mechanical performance of thearticle and corrosion resistance of the surface of the article has notbeen adequately addressed. In attempting to obtain the mechanicalproperties required for the aluminum alloy body of an article, it ispossible to affect the surface of the aluminum alloy in a manner suchthat the aluminum oxide (anodized) layer does not form a properinterface with the aluminum alloy. This creates a porosity, for examplegaps between the aluminum oxide layer and the underlying aluminumsurface. We have determined that it is particularly difficult to form aprotective anodized coating over a weld joint. The porosity andimpurities present at a conventional weld joint interfere withanodization of the aluminum present at the weld joint surface. Thisporosity promotes a breakdown in the protective aluminum oxide layer,leads to particle formation and a constantly accelerating degradation ofthe protective aluminum oxide film.

Not only is there significant expense in equipment maintenance andapparatus replacement costs due to degradation of the protectivealuminum oxide film, but if a susceptor, for example, developssignificant surface defects, these defects can translate through asilicon wafer atop the susceptor, creating device current leakage oreven short. The loss of all the devices on a wafer can be at a cost ashigh as $50,000 to $60,000 or more.

It is readily apparent from the above discussion that there is a longstanding need for a method of producing semiconductor apparatuscomponents which have a complex shape (the component is not merely aflat plate, for example), which have adequate mechanical properties forthe intended application, and which are protected by an anodized coatingwhich is capable of withstanding a corrosive plasma environment.

SUMMARY OF THE INVENTION

We have discovered a method of producing a complex-shaped aluminum alloyarticle, where welding has been employed to form the article, where ananodized aluminum coating is produced over a surface of the articleincluding a weld joint, and where the anodized aluminum coating providesimproved performance over that previously known in the art, when exposedto a corrosive plasma environment.

The welding of elements of the aluminum alloy article to form a complexshape is carried out using frictional welding, or a similar techniquewhich permits welding without the migration of a significant amount ofimpurities contained in the aluminum alloy toward the weld joint. Asignificant amount is intended to mean an amount which wouldsignificantly harm the subsequent formation of an anodized aluminumoxide protective coating over an aluminum alloy surface including theweld joint. Significant harm refers to the shortening of the performancelifetime of the anodized aluminum article. For example, prior to thepresent invention, the performance lifetime of an anodized weldedaluminum article was shortened by about 80% compared to the lifetime ofan anodized non-welded article.

In one embodiment, the particular aluminum alloy which is used to formthe body of an article of apparatus may be forged, extruded or rolled,and should have the following composition by weight %: A magnesiumcontent of about 0.1% to about 6.0%, with a mobile impurity atom contentof less than 2.0%. Mobile impurity atoms include metal atoms other thanmagnesium, including transitional metals, semiconductors, and atomswhich form semiconductor compounds. Mobile impurity atoms of particularinterest include silicon, iron, copper, chromium, titanium and zinc.When the article of apparatus is to be used at operational temperatureswhich are greater than about 250° C., the magnesium content of thealuminum article should range between about 0.1% by weight and about1.5% by weight of the article and the mobile impurity atom contentshould be less than about 0.2% by weight.

In a second embodiment, which has provided excellent results, which willbe subsequently described in detail, a particular aluminum alloy is usedto form the body of a semiconductor apparatus article. The raw aluminumalloy stock may be forged, extruded or rolled. The aluminum alloy shouldhave the following composition (in addition to aluminum) by weight %: amagnesium concentration ranging from about 3.5% to about 4.0%, a siliconconcentration ranging from 0% to about 0.03%, an iron concentrationranging from 0% to about 0.03%, a copper concentration ranging fromabout 0.02% to about 0.07%, a manganese concentration ranging from about0.005% to about 0.015%, a zinc concentration ranging from about 0.08% toabout 0.16%, a chromium concentration ranging from about 0.02% to about0.07%, and a titanium concentration ranging from 0% to about 0.01%, withother single impurities not exceeding about 0.03% each and other totalimpurities not exceeding about 0.1%. In some instances, depending onwhat the impurities are, the other total impurities may be permitted torise to about 0.2% by weight. In addition, the aluminum alloy isrequired to meet a particular specification with respect to particulatesformed from mobile impurities. Of the particulate agglomerations ofimpurity compounds, at least 95% of all particles must be less than 5 μmin size. Five (5) % of the particles may range from 5 μm to 20 μm insize. Finally, no more than 0.1% of the particles may be larger than 20μm, with no particles being larger than 40 μm. This high purity aluminumalloy is referred to as LP™ alloy hereafter. LP™ is a trademark ofApplied Materials, Inc. of Santa Clara, Calif.

After welding of the elements of the aluminum alloy article to form acomplex-shaped part, the aluminum alloy may optionally be stressrelieved at a temperature of about 330° C. or less, prior to creation ofthe aluminum oxide protective film over the article surface. The end useapplication for the part determines whether stress relief is necessary.A side benefit of the heat treatment process is that it providesadditional hardening of the alloy, despite prior art assertions to thecontrary with respect to aluminum alloys. It is very important that whenheat treatment is used, the heat treatment is carried out using lowerpeak temperatures than commonly recommended for aluminum alloys.Employment of a peak stress relief temperature of less than about 330°C. will ensure the desired material properties of the alloy with respectto grain structure, non-aluminum metal distribution properties, andmechanical properties in the article produced. By controlling the grainsize of the aluminum alloy during stress relief, and the distribution ofimpurities within the alloy, it is possible to avoid or at leastsignificantly reduce the formation of impurities near the surface of analloy article, which impurities interfere with the formation of ananodized aluminum oxide coating on the surface of the article. Thisensures the formation of a uniform anodized coating over the entirearticle surface, including any weld joints in the article. This methodof stress relief works particularly well in combination with the LP™alloy.

An anodized aluminum oxide protective film is typically applied using anelectrolytic oxidation process. Generally, the article to be anodized isimmersed as the anode in an acid electrolyte and a DC current isapplied. On the surface, the aluminum alloy is electrochemicallyconverted into a layer of aluminum oxide.

Prior to the anodization process, it is important to chemically cleanand polish the aluminum alloy surface. The cleaning may be carried outusing a method known to those skilled in the art. A particularlyeffective cleaning may be carried out by contacting the surface of thealuminum article with an acidic solution including about 60% to 90%technical grade phosphoric acid, having a specific gravity of about 1.7and about 1%-3% by weight of nitric acid. The article temperature duringcleaning is typically in the range of about 100° C., and the time periodthe surface of the article is in contact with the cleaning solutiontypically ranges from about 30 to about 120 seconds. Often, the cleaningprocess is followed by a deionized water rinse.

The subsequent anodization of the aluminum alloy surface to create analuminum oxide surface may be carried out using anodization techniquesknown in the art. A particularly good anodized protective coating may beobtained electrolytically in a water-based solution comprising 10% to20% by weight sulfuric acid and about 0.5% to 3.0% by weight oxalicacid. The anodizing temperature is typically set within a range fromabout 5° C. to about 25° C., and often within a range from about 7° C.to about 21° C. The article to be “anodized” serves as the anode, whilean aluminum sheet of standard 6061 serves as the cathode. Generally,during the electrolytic oxidation process the current density, inAmps/Square Foot (ASF) in the electrolytic bath, ranges from about 5 ASFto less than 36 ASF. Further, the “barrier layer” thickness at the baseof the aluminum oxide film may be controlled by the operating(anodization) voltage, which typically ranges from about 15 V to about30 V.

BRIEF DESCRIPTION OF THE DRAWINGS

A better understanding of the invention is obtained when the followingdetailed description is considered in conjunction with the followingdrawings in which:

FIG. 1A illustrates a schematic cross-sectional Scanning ElectronMicroscope view of an electron beam welded structure 100, where LP™aluminum alloy 102 is welded using a 4047 aluminum-containing fillermaterial 104. One section of the weld 103 exhibited cracking 106. Ananodized coating is present on the upper surface of the welded structure100. The anodized coating 108 present on the LP™ alloy 102 boundary 107is relatively thick. The anodized coating 110 present at the boundary105 with weld joint interface 103, and at the boundary 109 with 4047filler material 104 is relatively thin, with impurity particulates 112present at and, in some instances, extending through anodized coating110.

FIG. 1B illustrates an enlarged view of the structure shown in FIG. 1Aat the area indicated. This shows the 4047 filler material boundary 109with thin anodized coating 110 at the indicated location. This enlargedview shows better the particulates (impurity agglomerates) 112 which arepresent throughout the 4047 filler material 104, with some of theparticulates 112 extending up through anodized coating 110 at locations114.

FIG. 2A shows a schematic cross-sectional view of a preform chamberliner body section 200 having sidewalls 202 and a welding extension 204.The liner body section is in the form of an extruded tube of LP™aluminum alloy, where the centerline of the tube is marked 206.

FIG. 2B shows a schematic top view of the preform chamber liner bodysection 200 of FIG. 2A, where 207 represents the centerline.

FIG. 2C shows a schematic cross-sectional view of a preform top platewith flange 220 which is to be friction welded to preform chamber linerbody section 200. Top plate with flange 220 includes a base 208, aflange 210, a lip 212, and a centerline 216.

FIG. 2D is a schematic top view of the top plate with flange 220 of FIG.2C, where 217 represents the centerline.

FIG. 2E shows a schematic cross-sectional view of the combined extrudedtube structure 200 and the top plate with flange 220 as they are linedup prior to friction welding, where centerline 206 of extruded tubestructure 200 is lined up with centerline 216 of top plate with flange220. It is the welding extension 204 of extruded tube structure 200 andthe welding extension 212 of top plate with flange 220 which will besacrificed, at least in part, during the frictional welding process andwhich will be squeezed out as flash to provide a clean fresh surface atthe weld interface (not shown).

FIG. 3A illustrates a cross section of a test coupon 300 taken from afriction welded process vessel liner (not shown in its entirety). Thealloy used to fabricate the liner was LP™. The chamber body liner(extruded tube) structure 302 was friction welded to top plate withflange 306 (machined from a flat plate extruded stock), with the weldline being in the area 304.

FIG. 3B illustrates the microstructure 302 m of the aluminum grains inthe chamber body liner (extruded tube) structure 302, which aluminumgrains were undisturbed by the welding process.

FIG. 3C illustrates the microstructure 304 m of the aluminum grains inthe area of the frictional weld 304.

FIG. 3D illustrates the microstructure 306 m of the aluminum grains inthe top plate with flange 306, which aluminum grains were undisturbed bythe welding process.

FIG. 4 illustrates a test coupon 400 taken from the friction weldedliner described with reference to FIGS. 3A through 3D. The test couponwas machined for bending and consisted of “dog ears” 402 and 406.Bending was carried out with friction welded joint 404 at the center ofthe bend.

FIG. 5A illustrates a cross section of a test coupon 500 taken from thesame friction welded process chamber liner as that discussed withreference to FIG. 3A, after cleaning and application of an anodizedaluminum oxide protective film.

FIG. 5B illustrates the thickness of anodized coating 502 a overexterior surface (boundary) 503 of chamber body liner structure 502.

FIG. 5C illustrates the thickness of anodized coating 504 a over theexterior surface (boundary) 505 of weld joint area 504.

FIG. 5D illustrates the thickness of anodized coating 506 a over theexterior surface (boundary) 507 of top plate with flange 506.

FIG. 6 shows welding process variables as a function of time for thewelding of a prototype chamber liner which exhibited excellent weldingresults.

FIG. 7 shows welding process variables as a function of time for thewelding of another prototype chamber liner which exhibited excellentwelding results.

DETAILED DESCRIPTION OF THE INVENTION

As a preface to the detailed description, it should be noted that, asused in this specification and the appended claims, the singular forms“a”, “an”, and “the” include plural referents, unless the contextclearly dictates otherwise.

In the past, complex-shaped semiconductor processing apparatus parts anddevices were generally fabricated by machining from a block of aluminumalloy. The huge ingots or blocks from which the apparatus parts weremachined had inherent porosity. The porosity was evident on the machinedsurface of chamber liners and other components which were machined fromthe ingots, and the porous surface could not be reliably anodized. Apoor anodized aluminum oxide protective layer on the surface of anapparatus part affected the corrosion resistance of the part uponexposure to a corrosive plasma. One source of aluminum alloy which had acontrolled grain structure and which was superior in terms of porosity(relative to the ingot) was extruded stock, such as an extruded tube.Although it was desirable to use an extruded tube for the sidewalls ofthe liner, there was the problem of how to fasten a top plate withflange to the sidewalls. We welded an extruded tube to a flanged plateand formed an anodized coating over the exterior surface of the weldedstructure. The area of the liner in which the weld joint was present wasdifficult to anodize. We tried several different methods of welding,including laser welding, electron-beam welding, and TIG welding, forexample, but not by way of limitation. Use of fusion welding techniquessuch as these results in heat affected areas and melting of filler, withcooling and recrystallization, so that the microstructure in the area ofthe weld is different from the microstructure in the parent material.None of these welding methods enabled application of a satisfactoryanodized coating over the area of the weld joint. To make matters worse,a semiconductor substrate tends to be positioned toward the top of aplasma processing chamber, so that the welded area of a chamber liner isexposed to the highest density plasma. As a result, it is this portionof the liner where the anodized coating needs to be particularlycorrosion resistant.

FIG. 1A illustrates a schematic cross-sectional SEM view of a typicalelectron-beam-welded structure 100, where LP™ aluminum alloy 102 iswelded using 4047 filler material 104. One section of the weld 103exhibited cracking 106. An anodized coating was formed on the uppersurface of the welded structure 100. The anodized coating 108 present atthe boundary 107 with LP™ alloy 102 was relatively thick, and would beexpected to provide adequate protection for underlying LP™ alloy 102when surface 107 is exposed to a corrosive plasma. However, the anodizedcoating 110 formed at boundary 105 in the area of weld interface 103 andat boundary 109 in the area of 4047 aluminum-based filler material 104was particularly thin, indicating questionable performance when exposedto the corrosive plasma. In addition, a close inspection of the anodizedcoating 110 formed at boundary 109 over the 4047 aluminum alloy 104showed, as illustrated in FIG. 1B, that impurity particulates 112 werepresent in large numbers near boundary 109 and extended through anodizedcoating 110 in some instances 114.

Based on our observations regarding welding processes which involved theuse of filler materials during weld formation, or which involveddiffusion processes where the aluminum alloy must be at or near meltcondition, we concluded that the surfaces presented in the welded partwere not conducive to the formation of a reliable anodized coating.

There are some high purity aluminum alloys known in the art which wereuseful in providing corrosion resistance with respect tohalogen-containing plasma. These alloys are described in U.S. Pat. No.5,756,222 to Bercaw et al., issued May 26, 1998, and titled:“Corrosion-Resistant Aluminum Article For Semiconductor ProcessingEquipment”. This reference discloses an article of manufacture useful insemiconductor processing which includes a body formed from a high purityaluminum-magnesium alloy having a magnesium content of about 0.1% toabout 1.5% by weight, either throughout the entire article or at leastin the surface region which is to be rendered corrosion-resistant, and amobile impurity atom content of less than 0.2% by weight. Mobileimpurity atoms are defined as consisting of metal atoms other thanmagnesium, transitional metals, semiconductors, and atoms which formsemiconductor compounds. Mobile impurity atoms particularly namedinclude silicon, iron, copper, chromium and zinc. The high purityaluminum-magnesium alloy may be overlaid by a protective film which isresistant to abrasion, such as an aluminum oxide or aluminum nitridefilm. The subject matter disclosed in this patent is hereby incorporatedby reference in its entirety.

U.S. Pat. No. 5,811,195 to Bercaw et al., issued Sep. 22, 1998, andtitled: “Corrosion-Resistant Aluminum Article For SemiconductorEquipment”, further discloses that the magnesium content of the aluminumarticle may be in the range of about 0.1% to about 6.0% by weight of thealuminum article. However, for operational temperatures of the articlewhich are greater than about 250° C., the magnesium content of thealuminum article should range between about 0.1% by weight and about1.5% by weight of the article. In addition, an article is described inwhich the mobile impurities other than magnesium may be as high as about2.0% by weight in particular instances. One instance is when there is afilm overlying the exterior region of the article body, where the filmcomprises aluminum oxide or aluminum. Another instance is where there isa magnesium halide layer having a thickness of at least about 0.0025microns over the exterior surface of the aluminum article. The subjectmatter disclosed in this patent is hereby incorporated by reference inits entirety.

Cabot Corporation, along with other corporations have offered a highpurity aluminum alloy designated C276 for general sale for more than 10years. This high purity aluminum alloy chemical composition may beuseful in the present invention. However, the specification for the C276alloy includes a particulate size and distribution limitation withrespect to impurity compounds and agglomerations which may presentproblems regarding anodization of the alloy surface.

The Bercaw et al. patents previously mentioned are assigned to theassignee of the present invention, and additional work had been doneregarding high purity aluminum alloys subsequent to the discoveries madeby Bercaw et al. In particular, a new high purity aluminum alloy meetingparticular requirements for impurity atoms and for particulate sizeranges of accumulations or agglomerates of impurity atoms and compoundsthereof has subsequently been developed. The new aluminum alloy LP™composition is described in U.S. patent application Ser. No. 10/07/869filed Feb. 8, 2002, and assigned to the assignee of the presentinvention. The content of that application is hereby incorporated byreference in its entirety. The LP™ alloy includes impurities in thefollowing ranges by weight %: magnesium at a concentration ranging fromabout 3.5% to about 4.0%, silicon at a concentration ranging from 0% toabout 0.03%, iron at a concentration ranging from 0% to about 0.03%,copper at a concentration ranging from about 0.02% to about 0.07%,manganese at a concentration ranging from about 0.005% to about 0.015%,zinc at a concentration ranging from about 0.08% to about 0.16%,chromium at a concentration ranging from about 0.02% to about 0.07%, andtitanium at a concentration ranging from 0% to about 0.010%, with othersingle impurities not exceeding about 0.03% each and other totalimpurities not exceeding about 0.1%. The alloy composition measurementwas made by Sparking method for GDMS or by Molten method for GDMS.

In addition to the compositional limitations, applicants required thefollowing additional specifications with respect to the LP™ aluminumalloy. Of the particulate agglomerations of impurity compounds, at least95% of all particles must be less than 5 μm in size. Five (5) % of theparticles may be larger than 5 μm but less than 20 μm in maximumdimension. Finally, no more than 0.1% of the particles may be largerthan 20 μm, with no particles being larger than 40 μm. The analysistechnique used to determine particle size and size distribution wasbased on back scattered image analysis under the scanning electronmicroscope (SEM). The magnification was at 500×in order to assess theconstituent particles. The area of each image was about 150 μm×200 μm.The digital resolution was at least 0.2 μm/pixel. At least 40 imageswere taken at random from a sample area of 0.75 inch diameter in orderto obtain good assessment of various areas on the metal microstructure,to ensure meaningful statistical analysis. The back scattered imageswere digitally stored to provide for statistical analysis. The imageswere transferred to an image analyzer and the distribution of theparticles with a mean atomic number higher than that of Al (white in theimages) were detected and measured. The digital resolution allowed formeasurement of particles as small as 0.2 μm. The image analyzer used wasIBAS by Zeiss. Particle agglomerates were seen as precipitatedparticles. The parameters to determine the particle's size distributionwere: the diameter of the area equal circle φ=2× square root of (A/π),where A is the area of a particle. The class limits were as follows:0.2; 1; 2; 3; 4; 5; 20; 40. The number of particles in each class wasdetermined and then normalized to 100% for the total number of particlesmeasured.

The high purity LP™ aluminum alloy is expensive relative to standardaluminum alloys, making it important to conserve material. In addition,the machining cost of hogging out a solid ingot of material is high dueto the amount of machining time required, and the equipment andprogrammed machining instructions have to be reformulated for eachdifferent complex apparatus shape. We considered the use of an extrudedtube to obtain near net shape dimensions for the sidewalls of a chamberliner to very attractive, saving in both materials cost and machiningcost. However, the problem of how to weld the tube to a top plate withflange to form a chamber liner still remained.

To avoid the potential for particulate formation due to theagglomeration of mobile impurities in the area of the weld, we wanted toavoid a welding process which involved the use of a filler materialwhich would be a source of impurities. We also wanted to avoid the useof a welding process which required the melting or near melting of thealloy to be welded, which permits impurities to migrate, agglomerate,and move toward the surface of the article being welded. We selectedfriction welding as the welding process offering the best potential forachieving our goals.

The new problem became how to friction weld a large surface area of thekind required for a chamber liner, where the surface area to be weldedwas in the range of about 45 square inches (290 cm²). We were unable tofind a friction welding machine which was capable of handling such alarge surface area. We had to empirically develop a frictional weldingprocess which could be used to weld LP™ alloy structures of the sizerequired for process chamber liners and other large structures neededfor various semiconductor apparatus applications.

EXAMPLES Example One

For purposes of illustration, and not by way of limitation, theinvention will be described with respect to a method of producing aplasma process chamber liner of the kind used by assignee AppliedMaterials, Inc. in a number of different semiconductor processingsystems. For reference purposes, one skilled in the art may find ithelpful to refer to product information for the Applied Materials, Inc.e-Max™ chamber liner, e-Max™ cathode liner, and for the MXP⁺™ chamberliner for examples of apparatus which has been fabricated by us duringthe develoopment of the invention. The size of these process apparatuselements requires a weld surface area of 45 in², 35 in², and 45 in²,respectively.

The e-Max™ process chamber is used in the plasma etching of structureson semiconductor substrates. Plasma etching is frequently done usinghalogen-containing plasmas which are particularly corrosive. In view ofthe information we developed with respect to the properties of variousaluminum alloys, it was decided to fabricate a prototype LP™ aluminumalloy chamber liner. The liner component was fabricated from a flatplate flange joined to a cylindrical body. The cylindrical body wasformed as an extruded tube of the LP™ alloy. The problem was to join thetwo elements to form the complex-shaped chamber liner component. Afterconsiderable investigation, we determined that Manufacturing TechnologyIncorporated (MTI) of South Bend, Ind. was the sole U.S. domesticfabricator of friction and inertia welders with machine capability toaccommodate e-Max 200 mm liner components. The descriptions in thisexample pertain to a prototype chamber liner fabricated on frictionwelding tools available from this manufacturer.

The prototype LP™ alloy flange components were machined from plate stockand the cylindrical liner bodies were from extruded tube stock of thesame material. Flange and liner bodies were inertia welded at MTI underour direction, to validate an acceptable friction welding process, whileestablishing weld quality as a function of joining parameters.

With reference to FIG. 2A, this schematic cross-section illustrates apreform chamber liner body section 200 having sidewalls 202 and awelding extension 204. The thickness 204 T of welding extension 204 wasapproximately 1.05 inches (26.7 mm) and the height (length) 204 H wasabout 1.5 inches (38 mm). The exterior diameter D1 of the liner bodysection was about 14.65 inches (372 mm), and the interior diameter D2 ofthe liner body section (other than welding extension 204 ) was about11.45 inches (290 mm). Parallelism of the liner body section was within0.010 inch (0.25 mm), and concentricity was within 0.008 inch (0.20 mm).FIG. 2B is a top view of the preform chamber liner body section 200,where 207 represents the centerline which is shown as 206 on FIG. 2A.

With reference to FIG. 2C, this schematic cross-section illustrates apreform chamber liner top plate with flange 220 which is to be frictionwelded to preform chamber liner body section 200. Chamber liner topplate with flange 220 includes a base 208 having an exterior diameter D3of about 18.5 inches (470 mm), a flange 210 having an exterior diameterof about 20.0 inches (508 mm), and a welding extension 212, where thethickness 212T of the extension 212 at its end was about 1.05 inches(26.7 mm), and the height (length) 212H was about 1.12 inches (28 mm).Parallelism of the top plate section was within 0.005 inch (0.13 mm).Concentricity was within 0.008 inch (0.20 mm). FIG. 2D is a top view ofthe preform chamber liner top plate with flange 220, where 217represents the centerline which is shown as 216 on FIG. 2C.

FIG. 2E shows an assembly of the preform chamber liner body section 200and the preform chamber liner top plate with flange 220 prior tofriction welding.

The friction (inertia) welding process was carried out on MTI's 450-toninertia welding machine, which is comprised of a rotating headstock anda non-rotating tailstock. The desired weld energy was obtained bydriving an inertial mass fixed to the headstock to a specifiedrotational velocity. A PLC controller managed all machine functions asprogrammed to our specifications. The tailstock is programmed to moveaxially and is initially positioned to achieve a 0.1 inch gap betweenthe two components to be welded. On weld cycle initiation, the headstockis accelerated to the desired RPM, and the drive unit disengaged.Following a preprogrammed linear motion and axial force schedule, thetailstock is advanced, causing the two components to make contact,generate frictional heating and ultimately weld. As in most plasticdeformation welding processes, melting does not occur. Localized heatgeneration results in material property reduction and subsequent metalflow as upset force is applied. Heat effected material from the initialmating surfaces, along with oxides and contaminants are expelled in theflash. For purposes of reference in the description which follows, thefollowing definitions apply. Flash or weld flash typically refers to thematerial extruded from the weld interface due to the application ofaxial upset force. Flash will occur on both the outside diameter (O.D.)and the inside diameter (I.D.) of the assemblies in the weld area. Weldupset typically refers to the difference in length between the weldedand unwelded components. Upset results from the plastic displacement ofmaterial at the weld interface, and is dependent on weld energy,rotation velocity, and upset load. Faying surface typically refers tothe contacting surface of each of the two member which are to be joined.Upset force typically refers to the axial force applied at the fayingsurface to produce friction and upset displacement. Joint efficiencytypically refers to the ratio of the strength of a joint to the strengthof the base metal (expressed in percent).

With reference to “flash”, for example, FIG. 3A shows a coupon cut froma welded liner chamber body 300, where the chamber liner body section302 and the chamber liner top plate and flange 306 are welded togetherin the area 304, with flash 308 exiting from chamber body 300 and flash310 exiting from top plate and flange 306.

A preform chamber liner top plate with flange 220 was fabricated fromLP™ alloy plate measuring 4 inches thick by about 20 inches in diameter.One side of the plate was machined to produce the 1.5 inch high weldingextension 212, per our specification based on calculations particularlyapplicable to the LP™ alloy physical properties. The mating weldingextension 204 on the extruded preform of the chamber liner body section200 was machined flat and parallel to respective tolerances of 0.010inch. The first series of welding experiments included five part sets ofthe e-Max™ prototype chamber liners.

All of the five prototype chamber liners, fabricated from LP™ aluminumalloy, were welded by chucking the top plate with flange 220 in therotating headstock and the cylindrical liner body section 200 in thenon-rotating tailstock. Based on initial calculations and earlierexperiments, we had estimated that for joining a faying surface area ofabout 45 in² of LP™ aluminum alloy to form a “butt” joint between achamber body liner and a top plate with flange, the spindle velocity forwelding should range from about 350 RPM to about 750 RPM; the upset loadshould range from about 3,000 psi to about 4,000 psi; the total weldenergy should range from about 1,000 ft-lbs to about 3,500 ft-lbs; andthe upset should be about 0.9 inches to about 1.25 inches. The firstprototype chamber liner welding was carried out where the wall thicknessof the welding extensions was about 1.05 inches (26.7 mm), the weld areawas about 45 in², the RPM of the rotating headstock was 483, and theUp-set Pressure was 3,730 psi. The welding appeared to meet allrequirements, except that, due to equipment holding supportdifficulties, the upset was about 1.67 inches, and the chamber linerbody section 200 was flared where it joined the top plate with flange220. In view of the results, the other four prototype chamber linerswere fabricated under the same operating conditions, but with improvedholding support for the liner body section 200. The upset in inches forthe following four prototype chamber liners varied from about 0.99 toabout 1.08 inches.

The general metallography requirements for an acceptable welded partwere as follows: Examination of a section taken through the weld zone,using standard metallographic practices, must be uniform across thesmallest part joined, with examination in accordance with ASTM-E2,ASTM-E112, or AWS-B1.1.

With reference to FIGS. 3A through 3D, as discussed above, FIG. 3A showsa coupon cut from a welded liner chamber body 300, where the chamberliner body section 302 and the chamber liner top plate and flange 306are welded together in the area 304, with flash 308 exiting from linerchamber body 300 and flash 310 exiting from top plate and flash 306.FIG. 3B shows an SEM illustration of the aluminum grain structure(microstructure) for the extruded tube of LP™ alloy which was used tofabricate the liner chamber body section 302. The magnification in FIG.3B is about 50×. It is readily apparent in FIG. 3B that the grain sizeof grains 322 is generally uniform. FIG. 3C shows an SEM of the aluminumgrain structure for the weld zone area 304. The magnification in FIG. 3Cis about 50×. Clearly the grain size in the weld zone has been decreasedsubstantially. FIG. 3D shows an SEM illustration of the aluminum grainstructure for the parent plate stock of LP™ alloy which was used tofabricate the top plate and flange section 306. The magnification inFIG. 3D is about 50×. The grain size in the top plate and flange 306machined from the parent plate stock is also uniform.

Example Two

In another set of experiments two additional prototype LP™ alloy chamberliners were prepared in much the same manner as described above. Withrespect to Prototype No. 1, during the welding operation, the spindleRPM was about 500, the upset was 1.160 inch, and the upset force wasabout 4,000 lbs. FIG. 6 is a graph 600 showing process variables as afunction of time during the welding operation. In particular, axis 604shows a nominal value, while axis 602 shows time in seconds. Curve 610shows the spindle RPM, where the number on the nominal value axis 604 isequal to the spindle RPM×10⁻³; i.e, the number on the nominal value axis604 must be multiplied by 10³ to obtain the RPM. Curve 608 shows theupset pressure, where the number on the nominal value axis 604 is equalto the upset pressure in psi×10⁻⁴; i.e., the number on the nominal valueaxis 604 must be multiplied by 10⁴ to obtain the upset pressure in psi.Curve 610 shows the upset displacement in inches.

With respect to Prototype No. 2, during the welding operation, thespindle RPM was 470, the upset was 1.004 inch, and the upset force wasabout 4,000 lbs. FIG. 7 is a graph 700 showing process variables as afunction of time during the welding operation. In particular, axis 704shows a nominal value, while axis 702 shows time in seconds. Curve 710shows the spindle RPM×10⁻³; i.e. the number on the nominal value axis704 must be multiplied by 10³ to obtain the RPM. Curve 708 shows theupset pressure×10⁻⁴; i.e., the number on the nominal value axis 604 mustbe multiplied by 10⁴ to obtain the upset pressure in psi. Curve 710shows the upset displacement in inches.

The weld lines between the chamber liner body and the top plate withflange were barely discernable, even after exposure to an acetic etchsolution. Electro chemical etching was employed to resolve the grainstructure in the area of the weld, and the grain structure appearancewas very similar to that shown in FIG. 3C. The weld line was denoted bya band of very fine grains, the result of dynamic recrystallization.

A hardness survey was made for each prototype. In the case of PrototypeNo. 1, the hardness was measured starting at the weld line andprogressing at ⅛ inch intervals in the direction of the top plate andflange material. In the case of Prototype No. 2, the hardness wasmeasured starting at the weld line and progressing at ⅛ inch intervalsin the direction of the extruded liner chamber body section. Table 1below shows the results of the micro hardness survey.

TABLE 1 Location = weld line + Prototype No.1 Prototype No. 2 ⅛ inch ×number H_(k) (Knoop hardness) H_(k) (Knoop hardness) 0 88.2 78.5 1 88.275.2 2 82.0 75.3 3 85.5 75.4 4 85.5 75.3 5 81.8 75.1 6 77.0 75.0

It is readily apparent that the amount of inertial energy applied duringwelding minimally affects the hardness of the LP™ alloy in the area ofthe weld joint, with the hardness decreasing as the distance from theweld joint increases. In the case of the top plate material, it appearsto take about ¾ inch for the hardness to approach that of the parentmaterial. In the case of the extruded liner chamber body material, thehardness in the weld joint approaches that of the parent material withinabout the first ⅛ inch of travel away from the weld joint. In general,Table 1 shows that the work hardening at the weld interface is minimal.

In addition to evaluating the microstructure of the welded structure andthe hardness of the LP™ alloy in the area of the weld joint, we alsotested the elasticity of the weld as a means of testing the strength ofthe weld. FIG. 4 shows an example test specimen prepared from a weldedprototype liner which was welded in accordance with the processconditions provided above with respect to Prototype No. 2, after thespecimen was subjected to a bend test in accordance with ASTM methodE-290. Specimen “dog ear” 406 contains extruded chamber liner bodymaterial, while “dog ear” 402 contains top plate and flange material.The weld joint 404 showed no signs of cracking or void formation forbend angles as high as 90 degrees, when the bend radius=1 inch and thetest specimen thickness is about 0.3 inches.

To make the welded prototype liners plasma resistant, particularly tohalogen-containing plasmas, it is necessary to apply a protectivecoating which prevents abrasion of the liner surface as well as providedother beneficial functions. The surface of the liner to which thecoating is to be applied is generally cleaned prior to application ofthe coating. The cleaning method may be one known in the art. Aparticularly effective cleaning may be carried out by contacting thesurface of the aluminum article with an acidic solution including about60% to 90% technical grade phosphoric acid, having a specific gravity ofabout 1.7 and about 1%-3% by weight of nitric acid. The articletemperature during cleaning is typically in the range of about 100° C.,and the time period the surface of the article is in contact with thecleaning solution typically ranges from about 30 to about 120 seconds.If the cleaning time is too short, contaminants may remain on thearticle surface. If the cleaning time is too long, additional porositymay be created on the alloy surface and the interface formed between thealloy surface and the anodized film layer may degrade more rapidlyduring the process lifetime of the article. Often, the cleaning processis followed by a deionized water rinse first cleaned using a number ofdifferent methods known in the art.

An excellent coating which is compatible with the aluminum alloy surfaceis an electrochemically applied aluminum oxide protective film. Thisprotective film may be applied using any one of a number of methodsknown in the art. For example, Miyashita et al., in U.S. Pat. No.5,039,388, issued Aug. 13, 1991, describe a plasma forming electrodeused in pairs in a semiconductor processing chamber. The electrode isformed from a high purity aluminum or an aluminum alloy having a chromicacid anodic film on the electrode surface. The chromic acid anodizedsurface is said to greatly improve durability when used in a plasmatreatment process in the presence of fluorine-containing gas. Theelectrode is described as formed from a high purity aluminum such as JIS1050, 1100, 3003, 5052, 5053, and 6061 or similar alloys such as Ag—Mgalloys containing 2 to 6% by weight magnesium.

We used an internally developed method of electrochemically applying analuminum oxide protective film. The prototype chamber liner was immersedas the anode in an electrolyte bath comprised of a water-based solutionincluding 10% to 20% in a water-based solution comprising 10% to 20% byweight sulfuric acid and about 0.5% to 3.0% by weight oxalic acid. Theanodizing temperature is typically set within a range from about 5° C.to about 25° C., and often within a range from about 7° C. to about 21°C. The article to be “anodized” serves as the anode, while an aluminumsheet of standard 6061 serves as the cathode. Generally, during theelectrolytic oxidation process the current density, in Amps/Square Foot(ASF) in the electrolytic bath, ranges from about 5 ASF to less than 36ASF. Further, the “barrier layer” thickness at the base of the aluminumoxide film may be controlled by the operating (anodization) voltage,which typically ranges from about 15 V to about 30 V.

FIG. 5A illustrates a cross section of a test coupon 500 taken from thesame friction welded process chamber liner as that discussed withreference to FIG. 3A, after cleaning and anodization of the processchamber liner. The chamber body liner (extruded tube) structure 502 wasfriction welded to top plate with flange 506, with the weld line beingin the area 504, and with flash 508 being the residue from a weldingextension (not shown) on chamber body liner structure 502, which weldingextension was consumed during the frictional welding process. Flash 510is the residue from a welding extension (not shown) on the top platewith flange 506, which welding extension was consumed during thefrictional welding process. An anodized coating 502 a was formed overthe exterior surface 503 of chamber body liner structure 502; anodizedcoating 504 a was formed over the exterior surface 505 of weld jointarea 504; and anodized coating 506 a was formed over the exteriorsurface 507 of top plate with flange 506. FIG. 5B illustrates thethickness of anodized coating 502 a over exterior surface 503 of thechamber body liner (extruded tube) structure 502, where the averagethickness of the anodized coating was about 1.6 mils (0.0016 inch)(0.041 mm). FIG. 5C illustrates the thickness of anodized coating 504 aover the exterior surface 505 of weld joint area 504, where the averagethickness of the anodized coating increased slightly to about 1.7 mils(0.043 mm). FIG. 5D illustrates the thickness of anodized coating 506 aover the exterior surface 507 of top plate with flange 506, where theaverage thickness of the anodized coating was about 1.6 mils (0.041 mm).Not only was the anodized coating consistently reliable across the weldjoint and areas adjacent to the weld, but the coating was unexpectedlythicker over the weld joint than over the exterior surface 507 of topplate with flange 506. These coating thicknesses have not yet beenoptimized for the end use application, but are merely indicative of theuniformity of anodized coating which can be achieved when the method ofthe present invention is used to produce a complex-shaped welded partwith an anodized coating applied over at least a portion of the partincluding a welded area.

The above described exemplary embodiments are not intended to limit thescope of the present invention, as one skilled in the art can, in viewof the present disclosure, expand such embodiments to correspond withthe subject matter of the invention claimed below.

We claim:
 1. A method of producing a complex-shaped aluminum article foruse in semiconductor processing apparatus, wherein welding has beenemployed to form said article, comprising: selecting a high purityaluminum alloy with controlled particulate size and distribution andcontrolled composition of impurities present in said alloy, and weldingsaid high purity aluminum alloy using frictional welding, wherein avariation in hardness between a weld line and a welded parent materialis less than about 15%.
 2. A method in accordance with claim 1, whereina test specimen having a thickness in the range of about 0.3 inches,taken from said article in a manner which includes two parent materialsand a weld line between said parent materials, bend tested in accordancewith ASTM method E-290, having a bend radius of about 1 inch and a bendangle as high as 90 degrees, exhibits no signs of cracking or voidformation at said weld line.
 3. A method in accordance with claim 1 orclaim 2, wherein said high purity aluminum alloy exhibits controlledparticulate size and distribution of impurities such that at least 95%of all particles have a particle size of less than 5 μm, no more than 5%of said particles have a particle size ranging between 20 μm and 5 μm,and no more than 0.2% of said particles have a particle size rangingbetween 50 μm and 20 μm.
 4. A method in accordance with claim 3, whereinno more than 0.1% of said particles have a calculated effective diameterranging between 50 μm and 20 μm.
 5. A method in accordance with claim 4,wherein no more than 0.1% of said particles have a particle size rangingbetween 40 μm and 20 μm.
 6. A method in accordance with claim 3, whereinno more than 0.2% of said particles have a calculated effective diameterranging between 40 μm and 20 μm.
 7. A method in accordance with claim 1or claim 2, wherein said high purity aluminum alloy includes thefollowing impurity composition by weight %: magnesium ranging from about0.1% to about 6.0%, with other impurities selected from the groupconsisting of silicon, iron, copper, chromium, titanium and zinctotaling less than 2.0%.
 8. A method in accordance with claim 7, whereinsaid high purity aluminum alloy includes the following impuritycomposition by weight %: magnesium ranging from about 0.1% to about1.5%, with said other impurities totaling less than about 0.2% byweight.
 9. A method in accordance with claim 1 or claim 2, wherein saidhigh purity aluminum alloy includes the following impurity compositionby weight %: a magnesium concentration ranging from about 3.5% to about4.0%, a silicon concentration ranging from 0% to about 0.03%, an ironconcentration ranging from 0% to about 0.03%, a copper concentrationranging from about 0.02% to about 0.07%, a manganese concentrationranging from about 0.005% to about 0.015%, a zinc concentration rangingfrom about 0.08% to about 0.16%, a chromium concentration ranging fromabout 0.02% to about 0.07%, and a titanium concentration ranging from 0%to about 0.01%, with other single impurities not exceeding about 0.03%each and other total impurities not exceeding about 0.2%.
 10. A methodin accordance with claim 9, wherein said other total impurities do notexceed about 0.1%.
 11. A method in accordance with claim 3, wherein saidhigh purity aluminum alloy includes the following impurity compositionby weight %: magnesium ranging from about 0.1% to about 6.0%, with otherimpurities selected from the group consisting of silicon, iron, copper,chromium, titanium and zinc totaling less than 2.0%.
 12. A method inaccordance with claim 11, wherein said high purity aluminum alloyincludes the following impurity composition by weight %: magnesiumranging from about 0.1% to about 1.5%, with said other impuritiestotaling less than about 0.2% by weight.
 13. A method in accordance withclaim 3, wherein said high purity aluminum alloy includes the followingimpurity composition by weight %: a magnesium concentration ranging fromabout 3.5% to about 4.0%, a silicon concentration ranging from 0% toabout 0.03%, an iron concentration ranging from 0% to about 0.03%, acopper concentration ranging from about 0.02% to about 0.07%, amanganese concentration ranging from about 0.005% to about 0.015%, azinc concentration ranging from about 0.08% to about 0.16%, a chromiumconcentration ranging from about 0.02% to about 0.07%, and a titaniumconcentration ranging from 0% to about 0.01%, with other singleimpurities not exceeding about 0.03% each and other total impurities notexceeding about 0.2%.
 14. A method in accordance with claim 13, whereinsaid other total impurities do not exceed about 0.1%.
 15. A method ofproducing a plasma-resistant, complex-shaped aluminum article for use ina semiconductor processing apparatus, comprising: selecting a highpurity aluminum alloy with controlled particulate size and distributionand controlled composition of impurities present in said alloy; weldingsaid high purity aluminum alloy using frictional welding; andelectrochemically anodizing a surface of said complex-shaped aluminumarticle which includes a weld line, whereby a difference in thickness ofan aluminum oxide coating formed at said weld line compared with athickness of aluminum oxide coating formed at a surface of a parentmaterial unaffected by said welding is less than about 10%.
 16. A methodin accordance with claim 15, wherein said difference in thickness isless than about 7%.
 17. A method in accordance with claim 15, whereinsaid high purity aluminum alloy exhibits controlled particulate size anddistribution of impurities such that at least 95% of all particles havea particle size of less than 5 μm, no more than 5% of said particleshave a particle size ranging between 20 μm and 5 μm, and no more than0.2% of said particles have a particle size ranging between 50 μm and 20μm.
 18. A method in accordance with claim 17, wherein no more than 0.1%of said particles have a calculated effective diameter ranging between50 μm and 20 μm.
 19. A method in accordance with claim 18, wherein nomore than 0.1% of said particles have a particle size ranging between 40μm and 20 μm.
 20. A method in accordance with claim 17, wherein no morethan 0.2% of said particles have a calculated effective diameter rangingbetween 40 μm and 20 μm.
 21. A method in accordance with claim 15,wherein said high purity aluminum alloy includes the following impuritycomposition by weight %: magnesium ranging from about 0.1% to about6.0%, with other impurities selected from the group consisting ofsilicon, iron, copper, chromium, titanium and zinc totaling less than2.0%.
 22. A method in accordance with claim 21, wherein said high purityaluminum alloy includes the following impurity composition by weight %:magnesium ranging from about 0.1% to about 1.5%, with said otherimpurities totaling less than about 0.2% by weight.
 23. A method inaccordance with claim 15 or claim 16, wherein said high purity aluminumalloy includes the following impurity composition by weight %: amagnesium concentration ranging from about 3.5% to about 4.0%, a siliconconcentration ranging from 0% to about 0.03%, an iron concentrationranging from 0% to about 0.03%, a copper concentration ranging fromabout 0.02% to about 0.07%, a manganese concentration ranging from about0.005% to about 0.015%, a zinc concentration ranging from about 0.08% toabout 0.16%, a chromium concentration ranging from about 0.02% to about0.07%, and a titanium concentration ranging from 0% to about 0.01%, withother single impurities not exceeding about 0.03% each and other totalimpurities not exceeding about 0.2%.
 24. A method in accordance withclaim 23, wherein said other total impurities do not exceed about 0.1%.25. A method in accordance with claim 17, wherein said high purityaluminum alloy includes the following impurity composition by weight %:magnesium ranging from about 0.1% to about 6.0%, with other impuritiesselected from the group consisting of silicon, iron, copper, chromium,titanium and zinc totaling less than 2.0%.
 26. A method in accordancewith claim 25, wherein said high purity aluminum alloy includes thefollowing impurity composition by weight %: magnesium ranging from about0.1% to about 1.5%, with said other impurities totaling less than about0.2% by weight.
 27. A method in accordance with claim 17, wherein saidhigh purity aluminum alloy includes the following impurity compositionby weight %: a magnesium concentration ranging from about 3.5% to about4.0%, a silicon concentration ranging from 0% to about 0.03%, an ironconcentration ranging from 0% to about 0.03%, a copper concentrationranging from about 0.02% to about 0.07%, a manganese concentrationranging from about 0.005% to about 0.015%, a zinc concentration rangingfrom about 0.08% to about 0.16%, a chromium concentration ranging fromabout 0.02% to about 0.07%, and a titanium concentration ranging from 0%to about 0.01%, with other single impurities not exceeding about 0.03%each and other total impurities not exceeding about 0.2%.
 28. A methodin accordance with claim 27, wherein said other total impurities do notexceed about 0.1%.
 29. A method in accordance with claim 19, whereinsaid high purity aluminum alloy includes the following impuritycomposition by weight %: magnesium ranging from about 0.1% to about6.0%, with other impurities selected from the group consisting ofsilicon, iron, copper, chromium, titanium and zinc totaling less than2.0%.
 30. A method in accordance with claim 29, wherein said high purityaluminum alloy includes the following impurity composition by weight %:magnesium ranging from about 0.1% to about 1.5%, with said otherimpurities totaling less than about 0.2% by weight.
 31. A method inaccordance with claim 19, wherein said high purity aluminum alloyincludes the following impurity composition by weight %: a magnesiumconcentration ranging from about 3.5% to about 4.0%, a siliconconcentration ranging from 0% to about 0.03%, an iron concentrationranging from 0% to about 0.03%, a copper concentration ranging fromabout 0.02% to about 0.07%, a manganese concentration ranging from about0.005% to about 0.015%, a zinc concentration ranging from about 0.08% toabout 0.16%, a chromium concentration ranging from about 0.02% to about0.07%, and a titanium concentration ranging from 0% to about 0.01%, withother single impurities not exceeding about 0.03% each and other totalimpurities not exceeding about 0.2%.
 32. A method in accordance withclaim 31, wherein said other total impurities do not exceed about 0.1%.33. A method in accordance with claim 15, wherein said electrochemicalanodization is carried out by exposing said surface of said aluminumalloy to an electrolytic oxidation process during which said surface isimmersed as an anode in an acid electrolyte, with a cathode comprised ofan aluminum alloy, and wherein a DC current is applied, wherein saidacid electrolyte is a water-based solution comprising 10% to 20% byweight sulfuric acid and about 0.5% to 3.0% by weight oxalic acid,wherein said protective film is created at a temperature ranging fromabout 5° C. to about 25° C., and wherein an applied current density ofsaid DC current ranges from 5 A/ft² to 36 A/ft².
 34. A method inaccordance with claim 16, wherein said electrochemical anodization iscarried out by exposing said surface of said aluminum alloy to anelectrolytic oxidation process during which said surface is immersed asan anode in an acid electrolyte, with a cathode comprised of an aluminumalloy, and wherein a DC current is applied, wherein said acidelectrolyte is a water-based solution comprising 10% to 20% by weightsulfuric acid and about 0.5% to 3.0% by weight oxalic acid, wherein saidprotective film is created at a temperature ranging from about 5° C. toabout 25° C., and wherein an applied current density of said DC currentranges from 5 A/ft² to 36 A/ft².
 35. A method in accordance with claim17, wherein said electrochemical anodization is carried out by exposingsaid surface of said aluminum alloy to an electrolytic oxidation processduring which said surface is immersed as an anode in an acidelectrolyte, with a cathode comprised of an aluminum alloy, and whereina DC current is applied, wherein said acid electrolyte is a water-basedsolution comprising 10 % to 20% by weight sulfuric acid and about 0.5%to 3.0% by weight oxalic acid, wherein said protective film is createdat a temperature ranging from about 5° C. to about 25° C., and whereinan applied current density of said DC current ranges from 5 A/ft² to 36A/ft².
 36. A method in accordance with claim 19, wherein saidelectrochemical anodization is carried out by exposing said surface ofsaid aluminum alloy to an electrolytic oxidation process during whichsaid surface is immersed as an anode in an acid electrolyte, with acathode comprised of an aluminum alloy, and wherein a DC current isapplied, wherein said acid electrolyte is a water-based solutioncomprising 10% to 20% by weight sulfuric acid and about 0.5% to 3.0% byweight oxalic acid, wherein said protective film is created at atemperature ranging from about 5° C. to about 25° C., and wherein anapplied current density of said DC current ranges from 5 A/ft² to 36A/ft².